Radiotherapeutic apparatus involves the production of a beam of ionising radiation, usually x-rays or a beam of electrons or other sub-atomic particles. This is directed towards a cancerous region of the patient, and adversely affects the tumour cells causing an alleviation of the patient's symptoms. Generally, it is preferred to delimit the radiation beam so that the dose is maximised in the tumour cells and minimised in healthy cells of the patient, as this improves the efficiency of treatment and reduces the side effects suffered by a patient. A variety of methods of doing so have evolved.
One principal component in delimiting the radiation dose is the so-called “multi-leaf collimator” (MLC). This is a collimator which consists of a large number of elongate thin leaves arranged side to side in an array. Each leaf is moveable longitudinally so that its tip can be extended into or withdrawn from the radiation field. The array of leaf tips can thus be positioned so as to define a variable edge to the collimator. All the leaves can be withdrawn to open the radiation field, or all the leaves can be extended so as to close it down. Alternatively, some leaves can be withdrawn and some extended so as to define any desired shape, within operational limits. A multi-leaf collimator usually consists of two banks of such arrays, each bank projecting into the radiation field from opposite sides of the collimator.
It will of course be necessary to monitor the current actual position of the leaves in order to provide feedback and allow their position to be adjusted accurately. To date, two main methodologies have been employed in order to do so, namely;                Optical vision position sensing        Traditional positional sensing, for example potentiometers, encoders etc        
The approach to the problem of accurate leaf positional readout adopted to date by the applicant is outlined in FIG. 1. The solution is based on a vision system whereby a camera system “views” 84 different reflectors. 4 of these reflectors are reference markers, one in each corner of the viewable area, and 80 of these mark the individual position of each leaf of the two opposing banks of 40 leaves. The position of each leaf can therefore be calculated. The reflector is one having retro-reflective properties, i.e. light is reflected back along the same path as the incident light.
Thus, a camera 10 views the collimator leaves 12, 14, via a pair of tilt-adjustable mirrors 16, 18 which permit the camera to be located out of the radiation beam. A beam splitter 20 is placed in the optical path (between the two mirrors 16, 18 so that it is also out of the radiation beam) to allow a light projector 22 to illuminate the collimator leaves 12, 14 along the same optical path. A further mirror or mirrors 24 can be provided so as to locate the light projector (and/or other elements) in convenient locations.
Others tend to utilise traditional measurement methodologies. This involves measuring the position of each leaf by an individual sensor. A common design requirement is fault tolerance, or “single fault tolerance”, which implies that in order to assure the correct position of a single leaf, a secondary or back-up sensor must be used. This therefore doubles the number of position sensors required.
Various problems exist with both approaches. Optical methodologies and other current machine vision solutions require a high uniformity of illumination, which leads to difficulty in the recognition of valid reflectors. The leaf reflector material has only a limited lifetime, due to dirt and surface damage. The reflector material must be mounted with a high degree of accuracy. Stray light and/or stray reflections from internal reflections and/or reflections off a treatment table top can confuse the system. Finally, the retro-reflective properties of the leaf markers require the light source and the camera position to be in an optically identical location, to very tight tolerances, otherwise the shape and apparent brightness of the marker changes.
Traditional position measurement methodologies also suffer from serious difficulties. In particular, a very large number of sensors is required—a minimum two sensors per leaf to provide one primary readback and one backup readback. The degree of accuracy required and the quantity of sensors used conspire to mean that the system as a whole has a generally low degree of reliability. Further, there are difficulties in packaging the required quantity of sensors in a sufficiently compact design, and the sensors suffer from potentially poor reliability due to the radiation damage that inevitably results from their field of use.